The present invention generally relates to an automatic transmission in an automotive vehicle and, more particularly, to a hydraulic control for controlling the coupling timings of two friction coupling members, for example, a forward clutch and a coasting clutch, employed in the automatic transmission.
In the prior art automatic transmission, the forward clutch is generally fabricated to have a relatively great capacity because it is coupled during any one of the forward run and the drive whereas the coasting clutch is fabricated to have a relatively small capacity because it serves to transmit a torque in place of the forward clutch during the non-drive such as, for example, during the engine braking.
Since the coasting clutch is of small capacity as described above, it has a problem in that, a high load dash while it has been coupled at a timing earlier than the forward clutch, the high load tends to be imposed on the coasting clutch to such an extent as many result in the detrimental overheating of the coasting clutch. The relationship in coupling timing between the forward clutch and the coasting clutch is such that the coasting clutch must be coupled a predetermined delay time after the forward clutch has been coupled.
Hitherto, such a timing control has been carried out by the use of two orifices of different size. The timing control by the use of the orifices of different size has been, however, found disadvantageous in that an accurate operation can hardly be achieved. Therefore, it has been proposed to provide the control with an extra timing valve (delay valve) for enabling the clutches to be coupled at different timings such as disclosed in, for example, U.S. Pat. No. 4,225,829.
However, the use of the extra timing valve has a problem in that, not only is the cost increased correspondingly, but also no delicate adjustment in interrelationship between the forward and coasting clutches can be achieved at the time of coupling.